1. Field of the Invention
The present invention generally relates to a duplexer and a duplexer fabrication method, and more specifically, to a high-integrated duplexer fabricated using an embedded printed circuit board and a duplexer fabrication method thereof.
2. Description of the Related Art
A duplexer is a representative example of elements, which compositely uses a filter. The duplexer splits signals transmitted/received through one antenna in a frequency division duplex (FDD) type communication system so that it can serve to efficiently share the same antenna.
The duplexer basically includes a transmitter filter and a receiver filter. The transmitter filter is a band pass filter for passing only a frequency to transmit, and the receiver filter is a band pass filter for passing only a frequency to receive. The duplexer can perform the signal transmission/reception through the single antenna by varying the frequencies passing through the transmitter filter and the receiver filter.
With the rapid spread of mobile appliances, such as cellular phones, demand for a small-sized and lightweight duplexer used in such an appliance has been increasing. Also, demand for a small-sized and lightweight filter has been increasing, which serves as the transmitter filter and the receiver filter as internal components of the duplexer. To implement the small size and the lightweight filter for high power, an FBAR (Film Bulk Acoustic Resonator) is usually used because it can be mass-produced at a minimum cost and can be implemented with a minimum size. The FBAR can obtain a high quality factor (Q) value that is the principal characteristic of the filter, can be used in a micro-frequency band, and can be implemented up to PCS (Personal Communication System) and DCS (Digital Cordless System) bands.
Since the difference between the frequencies of the signals transmitted/received through the transmitter filter and the receiver filter is small, the signals are quite sensitive to inter-signal interference. Accordingly, a duplexer having an improved performance can be fabricated by adding an isolation part that prevents the inter-signal interference and by isolating the transmitter filter and the receiver filter from each other. The isolation part implements a phase shifter using a capacitor and a resistor, and prevents the inter-signal interference by making the phase difference between the frequencies of the transmitted signal and the received signal 90°.
FIGS. 1A and 1B show the constructions of the duplexer fabricated by using the conventional FBAR.
FIG. 1A shows the duplexer fabricated in a manner that a transmitter filter 30 and a receiver filter 40 are prepared respectively, and bonded onto a printed circuit board (PCB) 10 with an isolation part 20 for isolating the two filters 30, 40 from each other the two filters 30 and 40 by using a hybrid.
The PCB 10 is a circuit board fabricated in a manner that various parts are densely embedded onto a flat board which is made of a phenol resin or an epoxy resin, and a circuit connecting the parts is densely integrated onto and fixed to the surface of the flat board.
The transmitter filter 30 and the receiver filter 40 are respectively connected to an external electrode through a wire bonding method. The transmitter filter 30 and the receiver filter 40 are packaged separately and coupled to each other. The isolation part 20, which is separately fabricated, is bonded onto the flat board in the same process to fabricate the duplexer. However, in case of the wire bonding, there is a problem that a performance of an element is deteriorated due to a loss in the wiring and an increase in parasitic components. In addition, the final size of the duplexer element is increased due to the packaging size of the respective filters 30, 40 and the size of the isolation part 20 bonded between the filters 30, 40. Hence, the duplexer is not suitable for use in the miniaturized appliance such as a cellular phone and its fabricating cost is increased.
FIG. 1B shows a duplexer built on a substrate using a Bragg type FBAR, which has a structure wherein a lower electrode 63, a piezoelectric layer 62, and an upper electrode 61 are deposited in order on a reflector layer 64 formed by depositing, in layers, materials having a large difference in acoustic impedance, as a transmitter/receiver filter. As shown in FIG. 1B, the receiver filter includes a serial resonator 60 and a parallel resonator 90, and the transmitter filter includes a serial resonator 80 and a parallel resonator 90, all of which are integrated onto one substrate 50. The Bragg type duplexer is fabricated on one substrate to realize a one-chip fabrication and has a strong structure. However, the Bragg type duplexer has problems in that it is difficult to accurately adjust the width of the respective layers, and the film is easily cracked due to the stress caused by the forming of the thick reflector layer. Furthermore, the Bragg type duplexer has a considerably lower Q value in comparison to a duplexer using an air gap type FBAR.
In addition, due to the absence of an isolation part to prevent interference between the transmitter filters 60, 70 and the receiver filters 80, 90, much noise may be generated.